In today's cellular systems the use of multiple antennas (e.g. MIMO) for transmission is becoming increasingly important. An antenna configuration or system can be designed with either correlated or uncorrelated antenna elements, or combinations thereof where some antenna elements are correlated and others uncorrelated.
To fully be able to exploit the potential of these multiple antenna systems the transmission from correlated antenna elements need to be aligned in phase. Such alignment may be referred to as antenna calibration. Antenna calibration is mostly important when an antenna configuration includes correlated antenna elements and for features that require well directed beams and when wideband precoding is preferred.
An example of an antenna configuration comprising correlated antenna elements is the correlated cross-pole, depicted in FIG. 1. The correlated cross-pole is one of the most attractive four-antenna eNB setups for LTE. The advantages of the correlated cross-pole stem mainly from the possibility of combining beam forming with dual-layer transmissions and a small physical form factor. The cross-polarized elements, or cross-poles, 102 and 104, illustrated in FIG. 1, each comprises two mutually uncorrelated cross-polarized antenna elements, 1+3 (102) and 2+4 (104), respectively. The antenna elements 1 and 2 are illustrated as dashed lines, and the antenna elements 3 and 4 are illustrated as solid lines in FIG. 1. The antenna elements, 1 and 2 (dashed line) have the same polarization and compose a first pair, A, of correlated antennas. The antennas, 3 and 4 (solid line) have the same polarization and compose a second pair, B, of correlated antennas.
Correlated antenna pairs have beam forming properties with beam directions dependent on the phase difference between transmissions from the antenna elements. For a single pair of correlated antennas, such as e.g. antenna 1 and 2 (pair A in FIG. 1), the main lobe or beam during transmission is pointing in the direction where the phases of the antenna signals are added constructively. By changing the phase of the signal emitted e.g. from one of the antennas in the pair, the main lobe direction will change.
One source of error in systems using correlated antenna pairs is timing differences between antenna branches. Such timing differences may be due e.g. to feeder length differences or delay differences in the radio chains. This type of error will henceforth be referred to as a delay error.
A delay error between the transmissions from correlated antennas in a pair, will result in a frequency dependent phase error, which in turn causes a frequency dependent beam direction. The frequency dependency could be expressed as Δφ=−2πΔfτ, where τ is the delay or timing difference between transmissions from the correlated elements. This is generally harmful for performance and becomes more critical with increased transmission bandwidth.
An example illustrated in FIG. 2 shows the variations of the received power of a transmission over a pair of correlated antennas in a certain direction, over a bandwidth of 20 MHz, with a timing difference of 65 ns between the correlated antennas. Another illustration can be seen in FIGS. 3a-b, where it is shown that a UE 302 located at a fixed position may experience constructive addition of the signals (i.e. being in the main lobe) in one part of the frequency band, and experience destructive addition, e.g. a null, in another part of the frequency band. Two antenna pairs, such as A and B illustrated in FIG. 1 may in general have independent delay errors.
Also some aspects of the phase errors themselves, even when they are not frequency dependent, or if a sufficiently small bandwidth is considered, need to be corrected. Such absolute phase errors change the beam direction. An error related to the absolute phase of an antenna element is here referred to as a “absolute phase error”. Such absolute phase errors may be a problem if the precoder choices are limited or if, as in FIG. 1, there are two correlated pairs and the precoders are designed for specific relations between the two beams. Consider one of the correlated pairs in FIG. 1, e.g. antenna pair A. If there is no delay error between the elements of pair A, the beam direction will be defined by the absolute phase error difference between the antennas and will not be frequency dependent (which, as previously mentioned, is the case for delay errors).
For transmission to a single receiver, the beams of antenna pairs A and B should preferably be aligned, i.e. have their maximum beam-forming gain in the same direction. This put requirements on the difference between the absolute phase error differences between pair A and B i.e. Pd=(P4−P3)−(P2P1), where PX=absolute phase error on antenna element x. The difference in absolute phase difference between two pairs of antennas, Pd, is commonly referred to as a “phase error difference”, and will also be referred to as such henceforth in this description.
Other phase and delay errors may occur between the 4 antennas illustrated in FIG. 1. However, the described phase error difference (1 value) and timing errors (2 values) described above are the most important regarding system performance impact with the antenna configuration according to FIG. 1.
Solutions within a transmitter for estimating phase affecting errors, such as the delay and phase error difference described above, often require extra hardware, e.g. dedicated for calibration purpose only, which is expensive and inefficient.